Baij Lambert, Chassouant Louise, Hermans Joen J, Keune Katrien, Iedema Piet D
Van 't Hoff Institute for Molecular Sciences, University of Amsterdam PO box 94720 1090GD Amsterdam The Netherlands
Rijksmuseum Amsterdam, Conservation and Science PO box 74888 1070DN Amsterdam the Netherlands.
RSC Adv. 2019 Nov 1;9(61):35559-35564. doi: 10.1039/c9ra06776k. eCollection 2019 Oct 31.
Although the concentration of carboxylic acid (COOH) groups is crucial to understand oil paint chemistry, analytical challenges hindered COOH quantification in complex polymerised oil samples thus far. The concentration of COOH groups is important in understanding oil paint degradation because it drives the breakdown of reactive inorganic pigments to dissolve in the oil network and form metal carboxylates. The metal ions in such an ionomeric polymer network can exchange with saturated fatty acids to form crystalline metal soaps (metal complexes of saturated fatty acids), leading to serious problems in many paintings worldwide. We developed two methods based on ATR-FTIR spectroscopy to accurately estimate the COOH concentration in artificially aged oil paint models. Using tailored model systems composed of linseed oil, ZnO and inert filler pigments, these dried oil paints were found to contain one COOH group per triacylglycerol unit. Model systems based on a mixture of long chain alcohols showed that the calculated COOH concentration originates from side chain autoxidation at low relative humidity (RH). The influence of increasing RH and ZnO concentration on COOH formation was studied and high relative humidity conditions were shown to promote the formation of COOH groups. No significant ester hydrolysis was found under the conditions studied. Our results show the potential of quantitative analysis of oil paint model systems for aiding careful (re)evaluation of conservation strategies.
尽管羧酸（COOH）基团的浓度对于理解油画颜料化学至关重要，但迄今为止，分析上的挑战阻碍了对复杂聚合油样中COOH的定量分析。COOH基团的浓度对于理解油画颜料的降解很重要，因为它会促使活性无机颜料分解，溶解在油网络中并形成金属羧酸盐。这种离聚体聚合物网络中的金属离子可以与饱和脂肪酸交换，形成结晶金属皂（饱和脂肪酸的金属络合物），这在全球许多画作中都导致了严重问题。我们开发了两种基于衰减全反射傅里叶变换红外光谱（ATR-FTIR）的方法，以准确估算人工老化油画颜料模型中的COOH浓度。使用由亚麻籽油、氧化锌和惰性填充颜料组成的定制模型系统，发现这些干性油画颜料每个三酰甘油单元含有一个COOH基团。基于长链醇混合物的模型系统表明，计算出的COOH浓度源于低相对湿度（RH）下的侧链自氧化。研究了相对湿度和氧化锌浓度增加对COOH形成的影响，结果表明高相对湿度条件会促进COOH基团的形成。在所研究的条件下未发现明显的酯水解。我们的结果显示了对油画颜料模型系统进行定量分析的潜力，有助于仔细（重新）评估保护策略。
